A recent growing popularity of Internet of Things (IoT) has accelerated the development of such major wearable devices as watches and glasses that allow for Internet access. Even in the fields of medicine and sports, wearable devices for constantly monitoring the user's physical state are increasingly demanded, and such technological development is expected to be further encouraged.
In the field of medicine, including an electrocardiogram for detecting an electric signal to measure the motion of the heart, use of wearable devices for monitoring the state of human organs by detecting extremely weak current has been examined. The electrocardiogram measurement is conducted by attaching an electrode coated with a conductive paste to a body, but this is a single (not continuous), short-time measurement. On the other hand, the above medical wearable device is aimed at monitoring the state of physical conditions for a few weeks. Accordingly, a bio-electrode used in a medical wearable device is required to make no changes in conductivity even in long-time use and cause no skin allergy. In addition to these, bio-electrodes must be light-weight and produced at low cost.
Medical wearable devices are classified into two types: direct body attachment and clothing attachment. One typical body attachment device is a bio-electrode formed of a hydrophilic gel containing water and electrolytes as ingredients of the above conductive paste (Patent Document 1). The hydrophilic gel, containing sodium, potassium, and calcium electrolytes in a hydrophilic polymer containing water, detects changes in ion concentration from the skin to convert the data into electricity. Meanwhile, one typical clothing attachment device is characterized by a method for using as an electrode a fabric including a conductive polymer, such as PEDOT-PSS (Poly-3,4-ethylenedioxythiophene-polystyrenesulfonate), and a silver paste incorporated into the fiber (Patent Document 2).
However, the use of the hydrophilic gel containing water and electrolytes unfortunately brings about loss of conductivity due to water evaporation in drying process. Meanwhile, the use of a higher ionization tendency metal such as copper can cause some users to suffer from skin allergy, as well as a conductive polymer such as PEDOT-PSS due to strong acidity.
By taking advantage of excellent conductivity, the use of electrode materials formed of metal nanowire, carbon black, or carbon nanotube has been examined (Patent Documents 3, 4, and 5). With higher contact probability, metal nanowires can conduct electricity in small quantities to be added. Nevertheless, metal nanowires, formed of a pointed thin material, may cause skin allergy. Likewise, carbon nanotubes can stimulate a living body. Although the carbon black is not as poisonous as carbon nanotube, it also stimulates the skin. Accordingly, even though these electrode materials themselves cause no allergic reaction, the biocompatibility can be degraded depending on the shape of a material and its inherent stimulation, thereby failing to satisfy both conductivity and biocompatibility.
Although metal films seem to function as an excellent bio-electrode thanks to extremely high conductivity, this is not always the case. Upon heartbeat, the human skin releases a sodium ion, a potassium ion, or a calcium ion, instead of extremely weak current. It is thus necessary to convert changes in ion concentration into current, which is what less ionized precious metals unfortunately fail to do efficiently. The resulting bio-electrode including the precious metal is characterized by high impedance and high resistance to the skin during electrical conduction.
Meanwhile, the use of a battery containing an ionic liquid has been examined (Patent Document 6). Advantageously, the ionic liquid is thermally and chemically stable, and the conductivity is excellent, providing more various battery applications. However, an ionic liquid having smaller molecular weight shown in Patent Document 6 unfortunately dissolves into water. A bio-electrode containing such an ionic liquid in use allows the ionic liquid to be extracted from the electrode by sweating, which not only lowers the conductivity, but also causes rough skin by the liquid soaking into the skin.
In addition, any bio-electrode fails to get biological information when it is apart from the skin. The detection of even changes in contact area can vary quantities of electricity traveling through the electrode, allowing the baseline of an electrocardiogram (electric signal) to fluctuate. Accordingly, in order to stably detect electric signals from the body, the bio-electrode is required to be in constant contact with the skin and make no changes in contact area. This requirement is satisfied, preferably by use of adhesive bio-electrodes. Moreover, elastic and flexible bio-electrodes are needed to follow changes in skin expansion and flexion.